1. Field of the Invention
The invention relates to earth drilling machines. More specifically, the present invention pertains to a drive head assembly for a drilling machine, and more specifically to a drive head assembly having a float box housing and removable float box housing insert.
2. Description of the Prior Art
U.S. Pat. No. 5,012,878 issued to Anderson discloses a drive head apparatus for driving the drill string of a rock boring machine. The apparatus comprises one or two drill string connectors having resilient means such as a coil spring acting therebetween, with floating bearings separably secured thereto, and with each bearing arrangement comprising respective cooperating, axially fixed bearing seating surfaces. Each floating bearing is in the form of a softer metal insert replaceable on the associated connector without replacement of the drill string connector. A spline cylindrical drive insert rotatably drives the drill string connector and is separable from the drive body in which it is seated and is axially reversible to swap wear ends, or is readily replaceable without need for replacement of the entire drive body.
U.S. Pat. No. 3,800,887 issued to West discloses a drive head assembly for an earth drilling machine having an output shaft which projects downwardly through an opening in the support frame. A crown gear at the lower end of the shaft mates with a complementary crown gear at the upper end of a drive head main body situated below the frame. A large bolt extends upwardly through an opening in an upper wall of the drive head main body. The bolt includes a large diameter head which seats against an inner surface of the upper wall of the drive head main body and a threaded stem portion which threads into a lower end portion of the shaft. A drive box is located inside of the drive head main body and is coupled thereto by means of splines in a manner permitting it to both float axially and swivel sideways.
Both of these prior art patents disclose drive head assemblies, and drilling machines employing drive head assemblies, wherein the drive head assemblies include a collet, or float box housing, which is unitary in construction. Thus, these unitary collets include splines which mate with complimentary splines on the drive head proper, a rounded shoulder which pivots and swivels within the drive head proper, and an interior portion that threadedly engages the drill string. This type of unitary collet must be replaced when one portion of it is excessively worn. Excessive wear usually first occurs at the point of threaded connection between the collet and the drill string, with the collet splines and collet shoulder still having useful life remaining at the time of collet replacement. Additionally, because high strength alloys are required for threaded connection of the collet and the drill string, the entire unitary collet must be comprised of these expensive materials.
A need thus exists for a drive head assembly of a drilling machine that has a collet of composite construction comprised of a collet member having splines and a pivotable shoulder, and a separable collet insert for threaded connection with the drill string, said collet insert being replaceable when worn without the replacement of the collet member.